1. Field of the Invention
The present invention relates to an angular encoder including a tensible tape scale having a scannable graduation and designed for taking circular angular measurements, and a tensioning device connected with at least one end of the tape scale for tensioning the tape scale to adjustably position the at least one end.
2. Description of the Prior Art
Systems of the above-described type are used, in particular, for taking angular measurements when large round tables are used, with the tensible tape scale being wound around the table circumference, and with the tensioning device being connected with both ends of the tape scale for tensioning the tape scale.
The exact positioning of the ends of the tape scale with tensioning device is very important because the ends represent an unstable point of the angular encoder. In order to achieve an adequately large angle resolution, the length of the grating period (i.e., in case of a periodical line graduation, the distance between adjacent lines defining the grating period) is usually several units of 10 xcexcm. The positioning of the tape scale ends should be effected with a precision in the above range, i.e., within several units of 10 xcexcm, in order to be able to achieve the angle resolution defined by the grating period of the tape scale.
The reference book of Alfons Ernst, xe2x80x9cDigitale Langen-und Winkelmesstechmikxe2x80x9d (Digital Linear and Angular Measurement Technology), Landsberg/Lech: Publishing House xe2x80x9cModem Industry,xe2x80x9d Third Edition (1998), p.p. 69/70, discloses tensioning of the tape scale ends with a turnbuckle having two shoes that provide for butt-joining of the two ends in the tensioned condition of the tape.
The German Publication 272 698A1 discloses an angular encoder with a steel tape scale the operational length of which extends only over a half of the circumference of the round table and has, at its both ends, extension members that are mounted on another half of the table and that provide for tensioning of the tape scale with tensioning elements. In the disclosed embodiment, the two ends do not form a butt-joint but rather form a gap therebetween. For scanning the steel tape scale, the angular encoder has two scanning heads arranged diametrically opposite each other and of which one head is always at the same half of the round table on which the tape scale is mounted.
An object of the present invention is to provide an angular encoder of the type described above which would insure a highly precise positioning of the tape scale ends with simple means.
The objects of the present invention, which will become apparent hereinafter, are achieved by providing a tensioning device connected with one or both ends of the tape scale for tensioning the tape scale to adjustably position the one or both ends, with the tensioning device including an actuation member, and a transmission mechanism for connecting the actuation member with the one or both ends of the tape scale so that the operation of the actuation member would result in a predetermined change of a position of the one or both ends of the tape scale which position change is determined by a transmission ratio of the mechanism.
The advantage of the present invention consists in that the transmission mechanism converts the movement of the actuation member of the tensioning device, which can be effected with a suitable tool engageable with the actuation member, in a much shorter movement of the end(s) of the tape scale. A simple mechanical mechanism permits to achieve a transmission ratio of 1:1000 as it will be explained in detail further below. The present invention permits to achieve the described adjustment precision of positioning of the tape scale ends, without the need for both the actuation member and the actuating tool to meet high precision requirements.
In accordance with a preferred embodiment of the present invention, the actuation member is connected, via the transmission mechanism, with both ends of the tape scale. In this case, a symmetrical positioning of both tape scale ends can be effected with the actuation member.
According to the present invention, the transmission mechanism contains a lever mechanism including a lever having a first arm associated with the actuation member, and a second arm associated with at least one end of the tape scale and having a length shorter than a length of the first arm, with the lever being pivotal about a turning knuckle or a flector.
The transmission mechanism includes two levers for connecting the actuation member with both tape scale ends, then, preferably, they are connected with a yoke provided, respectively, with two hinges about which the levers can pivot. When the hinges are formed as flectors, they can be integrated into the yoke so that the yoke, the hinges and the levers form a one-piece unit.
Alternatively, the two levers can be formed as separate components forming together a unit only upon mounting of the angular encoder. When it is advantageous to form the transmission mechanism as a one-part unit or a multi-part unit depends on a particular case, i.e., on a structural relationship of the particular system. E.g., it can depend on whether the axle of the round table, about the circumference of which the tape scale has to be put, has a free end so that the tape scale, together with the scanning device can be axially pushed thereon. In this case, the tensioning device is advantageously formed as a one-piece unit. In case, when there is no free end of the table axle available and, therefore, the tape scale with the tensioning device cannot be pushed thereover on the round table, preferably, the tensioning device is formed of several parts. The tensioning device, in this case, is formed into unit only after the tape scale has been mounted around the circumference of a round table.
According to the present invention, the transmission mechanism includes a force deflection element acted upon by an actuation member displaceable in a first direction and causing thereby a displacement of the force deflection element in a second direction extending at angle or transverse to the first direction, with the displacement of the force deflection element resulting in a change of position of at least one end of the tape scale. The force deflection element is connectable with one or both ends of the tape scale by one or two links, respectively, with the link(s) being formed by a lever(s) of a type discussed above.
Preferably, the force deflection element itself contributes to the required transmission ratio by being formed so that its displacement in a direction transverse to the operational direction of the actuation member is much shorter than the displacement of the actuation member acting on the force deflection element.
The force deflection element can be formed, e.g., as a wedge element operationally connectable with the actuation member. Alternatively, the force deflection element can comprise a deformable member connected with the actuation member and deformable in a direction transverse to the displacement direction of the actuation member.
The actuation member can be formed, e.g., as an eccentric or as a longitudinally displaceable member, in particular, as an adjusting screw.
The tensioning device according to the present invention can be used with both angular encoders with which both ends of the tape scale are butt-joined by the tensioning device, and angular encoders with which a gap remains between the tape scale ends. In the last case, the solution according to the present invention is particularly advantageous, as it permits to exactly adjust the distance between the two ends of the tape scale.
The tensioning device is advantageously so formed that it can be positioned in a recess formed in a round table about which the tape scale is placed. Further, the tensioning device has one or several support elements with which the tensioning device is supportable against support surfaces formed in the recess.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.